Polyetherimides are a class of high performance polymers that can be processed to make molded articles, fibers, films, foams, and the like. Polyetherimides further have high strength, toughness, heat resistance, modulus, and broad chemical resistance, and so are widely used in industries as diverse as automotive, telecommunication, aerospace, electrical/electronics, transportation, and healthcare. Polyetherimides have shown versatility in various manufacturing processes, proving amenable to techniques including injection molding, extrusion, and thermoforming, to prepare various articles.
A number of processes for making polyetherimides have been disclosed. Two processes which have been of particular interest are the so-called melt polymerization and solution polymerization processes. Solution polymerization is generally conducted by reacting an aromatic dianhydride and an organic diamine in an inert solvent at elevated temperatures to form an amide-acid polymer via ring opening of the anhydride by nucleophilic attack of the diamine. The polyamide-acid is then formed into a polyetherimide by removal of water, for example by azeotropic distillation.
Aromatic dianhydrides are thus important to the production of polyetherimides. The aromatic dianhydrides can be prepared using an exchange reaction between an aromatic bisimide and a substituted or unsubstituted phthalic anhydride. In addition to dianhydride, the exchange reaction often produces various by-products which result in decreased yields of the dianhydride.
Accordingly, there remains a need for an improved method for producing and isolating dianhydrides that can provide high yields and minimize by-product formation.